Nonvolatile memories retain their stored information after their power supply has been turned off, and have many applications in digital information systems, control apparatus, and other electronic systems. Although several forms of nonvolatile memories exist, each has serious shortcomings.
Over the last ten years, much work has been performed developing magnetic bubble memories. However, bubble memories remain complex, and require a large current drive for their operation. Additionally, the cost of bubble memory greatly exceeds that of other memory devices.
EPROMs and EEPROMs are capable of retaining information over the extended periods of time without losing data, but the charge, and therefore the data, within an EPROM or an EEPROM will eventually be lost. Furthermore, changing the information stored in an EPROM or an EEPROM cannot be performed rapidly. Static RAMs and dynamic RAMs, while not nonvolatile, can be maintained by a battery during periods when system power is turned off. However, batteries have a relatively short useful life, and thereby present the danger of losing valued information as the batteries wear out.
While important strides have been made in improving the storage capacity of random access memories, it is still desirable to increase the storage capacities further, especially in nonvolatile devices.
Thus, a need has arisen in the industry for a fast, nonvolatile memory having a high storage capacity.